Use Userspace-RCU for SMR
Instead of using our own QSBR implementation that @fanf wrote, we should use Userspace-RCU instead. It's well-tested and well-maintained and it contains more APIs than just RCU, especially the CDS (Concurrent Data Structures) API is going to be very useful. While we can certainly implement our own RCU-based list, queue, stack and hashtable, it's simply better and faster to use battle-tested version.